Technical Abstract:
The need for a nonthermal intervention technology that can achieve microbial safety without altering nutritional quality of liquid foods led to the development of the radio frequency electric fields (RFEF) process. Previously, we documented formation of surface blebs on Escherichia coli cells treated with RFEF. In this study, we used transmission electron microscopy (TEM) to investigate the effects of heat and RFEF treatments on E.coli K-12 cell membrane. Apple juice purchased from a wholesale distributor was inoculated with Escherichia coli K-12 at 7.8 log CFU/ml and then treated with heat (75 deg C, 5 min) or with RFEF operated at 55C and 25 kV/cm for 3.4 milliseconds at a flow rate of 540 ml/min. Treated samples were monitored for changes in cell membrane using Transmission Electron Microscopy (TEM) while intracellular leakage of biological active compounds was determined using bioluminescence ATP assay. Surviving cell populations was determined by plating 0.1 ml sample on Tryptic Soy Agar and Violet Red Bile Agar, with 5 ml overlay of the same agar containing 4-methylunbelliferyl-beta-D-glucuronide to determine the number of colony forming units (CFU/ml). Bacterial inactivation determined for heat treatment at 75 deg C for 5 min and RFEF treatment at 55 deg C averaged 2.8 log CFU/ml and 5.74 log CFU/ml, respectively. The injured E. coli populations before treatment averaged 4.5%. In heat treated samples, the injured population was 86%, while it was 46 % in samples treated with RFEF. TEM observation of treated samples showed variation in membrane damage of the bacteria. Injured membrane and discharge of intracellular bacteria organelles were observed only in RFEF treated E. coli cells. Also, extracellular Adenosine Triphosphate (ATP) of treated samples increased suggesting leakage of intracellular ATP materials of bacteria. Total ATP determined in heat treated samples was significantly (p < 0.05) different than values determined in RFEF treated samples at 55 deg C. The results of this study suggest that the mechanism of inactivation of RFEF is by disruption of the bacterial cell surface structure leading to membrane injury and leakage of intracellular biological active compounds.